In recent years, it has become common to transmit not only speech data but also a large amount of data such as still image data and moving image data along with the adoption of multimedia information in cellular mobile communication systems. In addition, studies have been actively carried out to achieve a high transmission rate using a wide radio band, Multiple-Input Multiple-Output (MIMO) transmission technology, and interference control technique in Long Term Evolution Advanced (LTE-Advanced).
In LTE-Advanced, an enhanced control channel region called Enhanced PDCCH (EPDCCH), which is an enhanced version of Physical Downlink Control CHannel (PDCCH) used for control signals, has been designed. EPDCCH is located in a resource region to which downlink data is assigned (i.e., Physical Downlink Shared Channel (PDSCH) region). Base stations can configure each terminal (sometimes called “User Equipment (UE)”) with a frequency resource (e.g., resource block (RB)) in a resource region in which EPDCCH is located (EPDCCH region) and transmit signals to the terminal. Thus, it is made possible to achieve transmission power control for control signals transmitted to a terminal located near a cell edge, or interference control for interference caused by a cell of the base station to another cell or interference control for interference caused by another cell to the cell of the base station due to the control signals to be transmitted.
Each RB has 12 subcarriers in the frequency domain and a width of 0.5 msec in the time domain. The unit of two RBs combined in the time domain is called an RB pair. More specifically, each RB pair has 12 subcarriers in the frequency domain and a width of 1 msec in the time domain. Moreover, when an RB pair represents a block of 12 subcarriers on the frequency axis, the RB pair may be simply called an RB. An RB pair is also called a Physical RB pair (PRB pair) on the physical layer. In addition, the unit defined by one subcarrier and one OFDM symbol is a resource element (RE).
In an EPDCCH region, the units defined by dividing each PRB pair into 16 resources are called Enhanced Resource Element Groups (EREGs), and a resource unit formed of four or eight EREGs is called an Enhanced Control Channel Element (ECCE). The number of ECCEs forming EPDCCH transmitting one set of control signals is called an aggregation level (AL). A plurality of ALs can be configured for EPDCCH (see, Non-Patent Literature (hereinafter, referred to as “NPL”) 1). In addition, an “EPDCCH candidate” is previously defined in each AL. The term, “EPDCCH candidate” as used herein refers to a candidate for a region to which control signals are mapped, and a plurality of EPDCCH candidates form a search space, which is a blind-decoding (monitoring) target of a terminal.
LTE-Advanced allows each terminal to be configured with a plurality of EPDCCH sets each formed of a set of ECCEs on which EPDCCH may be located (i.e., control information assignment candidate). Since the positions and number N of PRB pairs to be used are configured for each EPDCCH set, control signals are arranged more flexibly.
FIG. 1 illustrates an example of how EPDCCHs are mapped to resources. The ECCEs on which EPDCCHs are located are selected from the abovementioned plurality of EPDCCH candidates. In FIG. 1, EPDCCH #0 and EPDCCH #1 are each configured with aggregation level 1 (AL1) while EPDCCH #2 and EPDCCH #3 are configured with aggregation level 2 (AL2) and aggregation level 4 (AL4), respectively. In FIG. 1, EPDCCH #0 is located on ECCE #0, and EPDCCH #1 is located on ECCE #1, while EPDCCH #2 is located on ECCE #2 and ECCE #3, and EPDCCH #3 is located on ECCE #4, ECCE #5, ECCE #6, and ECCE #7. In addition, since each ECCE is located on four EREGs, the ECCE is divided into four pieces in FIG. 1. As illustrated in FIG. 1, in case of localized assignment, four EREGs are located on the same PRB pair, while four EREGs are located on different PRB pairs in case of distributed assignment.
Incidentally, as an EPDCCH assignment method, there are “localized assignment” in which EPDCCH is assigned locally to positions close to each other on the frequency band, and “distributed assignment” in which EPDCCH is assigned distributedly on the frequency band. “Localized assignment” is an assignment method used to obtain frequency scheduling gain and enables assignment of EPDCCH to a resource having good channel quality on the basis of channel quality information. “Distributed assignment” allows obtaining frequency diversity gain by distributing EPDCCH on the frequency axis. LTE-Advanced allows both of a search space for localized assignment and a search space for distributed assignment to be configured.
In LTE-Advanced, downlink (DL) assignment indicating DL data assignment and an uplink (UL) grant indicating UL data assignment are transmitted on PDCCH or EPDCCH. DL assignment reports that a resource in a subframe used to transmit this DL assignment is assigned to a terminal. Meanwhile, UL grant reports that a resource in a target subframe previously determined by the UL grant is assigned to a terminal.    NPL 1    3GPP TS 36.213 V11.1.0, “Physical layer procedures